1. Field of the Invention
The present invention relates generally to Phase Shift Masks (PSMs) for use within semiconductor manufacturing. More particularly, the present invention relates to methods and materials for manufacturing attenuated Phase Shift Mask (PSM) blanks and attenuated Phase Shift Masks (PSMs) which are readily inspected and optically aligned.
2. Description of the Related Art
Advances in semiconductor integrated circuit performance have traditionally derived from the simultaneous continuing decreases in integrated circuit device dimensions and the correlating decreases in dimensions of conductor elements which connect those integrated circuit devices. In order to optimally produce advanced integrated circuits having these decreased dimensions, it has traditionally been required that the wavelength of coherent light employed in photolithographic processes by which integrated circuit devices and conductor elements are fabricated be substantially smaller than the minimum dimension within the reticle through which those integrated circuit devices and conductor elements are printed. As the dimension of the smallest dimension within the reticle through which integrated circuit devices and conductor elements are printed approaches the wavelength of coherent light employed to print those integrated circuit devices and conductor elements, the resolution, exposure latitude and depth of focus of the printed device or element decreases due to aberrational effects of coherent light passing through openings of width similar to the wavelength of the coherent light.
Therefore, as semiconductor technology has advanced, there has traditionally been a corresponding decrease in wavelength of light employed in printing the features of integrated circuit devices and conductor elements. Presently, light sources employed in printing the features of integrated circuit devices and conductor elements of sub-micron geometries are in the visible and Near Ultra-Violet (NUV) wavelength regions. For integrated circuit device features and conductor element features in the sub-half micron range, light sources in the Deep Ultra-Violet (DUV) and x-ray wavelength regions have been proposed.
As an alternative approach for providing high resolution printed integrated circuit devices and conductor elements of dimensions similar to the wavelength of coherent light by which those integrated circuit devices and conductor elements are printed, there has recently been proposed the use of Phase Shift Masks (PSMs) in place of conventional reticles. In comparison with conventional reticles, Phase Shift Masks (PSMs) typically incorporate an additional layer, usually patterned, within the conventional chrome metal-on-glass reticle construction. The additional layer, which is commonly referred to as a shifter layer, has a thickness related to the wavelength of coherent light passing through the Phase Shift Mask (PSM). The differences in optical properties between conventional reticles and Phase Shift Masks (PSMs) derive from the differences in light paths passing through: (1) the transparent substrate which is employed in both a conventional reticle and a Phase Shift Mask (PSM), and (2) the shifter layer which is employed only within a Phase Shift Mask (PSM). Coherent light rays passing through the transparent substrate and the shifter layer have different optical path lengths and thus emerge from those surfaces with different phases. The interference effects of the coherent light rays of different phase provided by a Phase Shift Mask (PSM) form a higher resolution image when projected onto a semiconductor substrate, which higher resolution image has a greater depth of focus and a greater exposure latitude.
Typical constructions of a conventional reticle and a conventional Phase Shift Mask (PSM), along with light intensities and light amplitudes achieved with those constructions, are illustrated in FIG. 1 to FIG. 8. FIG. 1 illustrates a conventional reticle constructed from a transparent substrate 10 and a patterned metal layer 12, along with coherent light rays 14 incident upon the transparent substrate 10. FIG. 2 illustrates the light amplitude immediately after transmission of the coherent light rays 14 through the conventional reticle illustrated in FIG. 1. FIG. 3 illustrates the light amplitude on a substrate surface upon which is printed through a projection or similar process the pattern of the conventional reticle illustrated in FIG. 1. FIG. 4 illustrates the light intensity on the substrate surface corresponding to the light amplitude on the substrate surface illustrated in FIG. 3.
In comparison with the conventional reticle illustrated in FIG. 1, FIG. 5 illustrates a conventional Phase Shift Mask (PSM) which has a patterned transparent shifter layer 16 occupying alternating light transmitting regions of the patterned metal layer 12. The light amplitude and light intensity graphs of FIG. 6 to FIG. 8 correspond respectively to the light amplitude and light intensity graphs of FIG. 2 to FIG. 4. The resolution, depth of focus and exposure latitude of the light intensity achievable with the conventional Phase Shift Mask (PSM), as illustrated in FIG. 8, is dramatic in comparison with the corresponding resolution, depth of focus and exposure latitude achievable under equivalent illumination conditions with the conventional reticle, as illustrated in FIG. 4.
There are various locations where a transparent shifter layer may be incorporated within a conventional reticle to provide a conventional Phase Shift Mask (PSM). For example, P. Burggraaf in "Lithography's Leading Edge, Part I: Phase-shift Technology," Semiconductor International, February 1992, pp 44-45 discloses several conventional Phase Shift Mask (PSM) constructions where a blanket or patterned transparent shifter layer is incorporated either above or below the patterned metal layer in a conventional reticle construction. In addition, Okamoto, in U.S. Pat. No. 5,045,417 discloses several additional Phase Shift Mask (PSM) constructions, some of which incorporate a series of grooves formed within light transmitting regions of a conventional reticle substrate, which grooves serve a patterned transparent shifter layer function without the need for an additional patterned transparent shifter layer.
One of the more unique Phase Shift Mask (PSM) constructions is the attenuated Phase Shift Mask (PSM). The attenuated Phase Shift Mask (PSM) enhances the resolution, depth of focus and exposure latitude of the dark areas to be printed. A cross sectional diagram illustrating an attenuated Phase Shift Mask (PSM) construction, along with its corresponding light amplitude and light intensity graphs, is illustrated in FIG. 9 to FIG. 12. As shown in FIG. 9, the attenuated Phase Shift Mask (PSM) has formed upon a transparent substrate 10 a patterned semi-transparent shifter layer 18. The patterned semi-transparent shifter layer 18 is typically formed of an oxidized metal layer which provides a 180 degree phase shift to the coherent light rays 14. The thickness of the patterns within the patterned semi-transparent shifter layer 18 is defined by the equation: EQU d=.lambda./2(n-1)
where d equals the thickness of the patterns within the patterned semi-transparent shifter layer 18, .lambda. equals the wavelength of the coherent light rays 14 and n equals the index of refraction of the material from which is formed the patterned semi-transparent shifter layer 18.
Although Phase Shift Masks (PSMs) provide substantial potential for cost avoidance with regard to the need for new generations of semiconductor photolithographic processing equipment, the manufacturing of Phase Shift Mask (PSM) blanks and Phase Shift Masks (PSMs) is not entirely without problems.
For example, the semi-transparent shifter layers found within attenuated Phase Shift Mask (PSM) blanks and attenuated Phase Shift Masks (PSMs) are often formed of materials having properties such that optical inspection, alignment and repair of those attenuated Phase Shift Mask (PSM) blanks and attenuated Phase Shift Masks (PSMs) is often accomplished only with difficulty. In particular, while it is common in the art that attenuated Phase Shift Masks (PSMs) will be employed with exposure wavelengths corresponding to the Near Ultra-Violet (NUV) (i.e.: 365 nm) region and the Deep Ultra-Violet (DUV) (i.e.: 248 nm) region, it is also common in the art that the alignment and inspection of those attenuated Phase Shift Masks (PSMs) and attenuated Phase Shift Mask (PSM) blanks from which are formed those attenuated Phase Shift Masks (PSMs) will occur with Helium-Neon laser light at 623 nm.
Thus, although there may exist many materials from which may be formed blanket semi-transparent shifter layers within attenuated Phase Shift Mask (PSM) blanks and patterned semi-transparent shifter layers within attenuated Phase Shift Masks (PSMs), either of which are operational at either the Near Ultra-Violet (NUV) (i.e.: 365 nm) exposure region or the Deep Ultra-Violet (DUV) (i.e.: 248 nm) exposure region, such materials will often not simultaneously possess the desired optical properties at the 623 nm Helium-Neon laser light inspection and alignment wavelength region. In particular, it is often found that materials from which may be formed blanket and patterned semi-transparent shifter layers in attenuated Phase Shift Mask (PSM) blanks and attenuated Phase Shift Masks (PSMs) operational in the Near Ultra-Violet (NUV) (i.e.: 365 nm) and the Deep Ultra-Violet (DUV) (i.e.: 248 nm) wavelength regions will often have an undesirably high light transmission at the 623 nm Helium-Neon laser light wavelength which is employed to optically inspect and align those attenuated Phase Shift Mask (PSM) blanks and attenuated Phase Shift Masks (PSMs).
Thus, it is desirable in the art to provide attenuated Phase Shift Mask (PSM) blanks and attenuated Phase Shift Masks (PSMs) whose constructions are fully operational at Near Ultra-Violet (NUV) (i.e.: 365 nm) or Deep Ultra-Violet (DUV) (i.e.: 248 nm) exposure wavelengths and simultaneously readily inspected and aligned with Helium-Neon laser light at 623 nm. It is towards this goal that the present invention is directed.